envelope
n. countablen. a flat paper cover used for sending letters or documents through the mail. You slide the paper inside and seal the top edge to keep it safe.
n. a flat, rectangular paper container with a sealable flap, designed for the transmission of correspondence. In technical contexts, it refers to a surrounding structure or boundary, such as a gas layer around a star or the outer membrane of a virus.
She licked the envelope and stuck a stamp on it.
The lawyer handed over a thick white envelope containing the signed contracts and the keys to the property.
While the primary sense refers to postal stationery, engineers often speak of the flight envelope to describe the maximum speed and altitude limits within which an aircraft can safely operate.
PIE word *h₁én From French enveloppe. The engineering sense is derived from flight envelope. The verb is from the noun.
See envelop.
Commonly used with the verb 'seal' or 'open'. In the idiom 'push the envelope', it refers to testing the limits of what is possible.